Since we live in a three dimensional world, it is necessary to control
the attitude or orientation of a flying aircraft in all three
dimensions.
In flight, any aircraft will rotate about its
center of gravity,
a point which is the average location of the mass of the aircraft.
We can define a three dimensional coordinate system through the
center of gravity with each axis of this coordinate system perpendicular
to the other two axes.
We can then define the orientation of the aircraft by the amount of
rotation of the parts of the aircraft along these principal axes.

The yaw axis is defined to be perpendicular to the plane of the
wings with its origin at the center of gravity and
directed towards the bottom of the aircraft. A
yaw motion
is a movement of the nose of the aircraft from side to side.
The pitch axis is perpendicular to the yaw axis and is parallel to the
plane of the wings with its origin at the center of gravity
and directed towards the right wing tip. A
pitch motion
is an up or down movement of the nose of the aircraft.
The roll axis is perpendicular to the other two axes
with its origin at the center of gravity, and is directed
towards the nose of the aircraft. A
rolling motion
is an up and down movement of the wing tips of the aircraft.

In flight, the control surfaces of an aircraft produce
aerodynamic forces.
These forces are applied at the
center of pressure
of the control surfaces which are
some distance from the aircraft cg and produce
torques (or moments)
about the principal axes. The torques cause the aircraft to rotate. The
elevators produce a
pitching moment, the
rudder produces a
yawing moment, and the
ailerons produce a
rolling moment. The ability to vary the amount of
the force and the moment allows the pilot to maneuver or to
trim the aircraft.
The first aircraft to demonstrate active control about all three
axes was the Wright brothers'
1902 glider.